The present invention generally relates to an improved cylindrical interference fit for synthetic polymeric constructed materials, and more particularly, to a smooth surfaced shaft and an internally ridged recess that produces a three-point interference fit useful for fastening together separate interconnecting electrical sectional terminal blocks.
Interconnecting electrical sectional terminal blocks, or plug-in modules, molded from electrically insulating materials such as synthetic polymeric materials, are known and used in which a plurality of separate blocks are fastened or assembled to one another to accommodate a plurality of electrical circuits. General prior art practice in synthetic polymeric materials typically favors cylindrical parts, such as a pin and a mating hole or receptacle to produce a cylindrical interference fit for joining and fastening the blocks into assemblies or circuits. Usually, the classes of interference fits employed for this purpose are either loose fits, i.e., sliding and locational fits, or force fits.
When loose fits in synthetic polymeric materials are utilized, additional fastener components, for example, hardware in the form of bolts, nuts, washers, etc., are frequently necessary to aid in fastening the separate blocks into more or less permanent assemblies. However, the use of additional components has several drawbacks, in that, their use is often costly and may entail tedious time consuming assembly. Moreover, the use of loose fits may also not allow a desired accuracy in locating interconnecting parts associated with the blocks intended to be assembled.
When force fits in synthetic polymeric materials are employed, the external cylindrical surface of the pin is normally constructed to be either smooth or to have multiple serrations thereon. Similarly, the internal circular surfaces of the recesses in synthetic polymeric materials are also constructed to be either smooth or to have a plurality of serrations therein. However, when the surfaces of the pin and hole are both smooth failure often occurs. That is, the pin, hole, or member itself may break or crack during assembly or disassembly due to pressing forces required to produce the force fit with magnitudes too great for the material utilized. Moreover, any misalignment in the interconnecting parts (due to variances in tolerances from the permitted dimensions of these parts required to permit the desired fit) frequently either increase the frequency of failure, or necessitate refitting or remanufacturing the members and/or their related interconnecting parts.
Although practice of constructing the external surfaces of the pins with multiple serrations thereon reduces the occurrence of failure by, in effect, putting lesser amounts of material in interference between parts, and thus diminishing the magnitude of pressing forces, this practice requires additional and often special tooling and/or machining to manufacture the serrated pin.
In some prior art plastic material devices, that utilize multiple serrations within the internal surfaces of the holes, the serrations are often mashed flat during the insertion of a smooth surfaced shaft therein, which in effect, creates an ordinary force fit that is subject to periodic incidences of failure. Further, additional and costly tooling and/or machining is required to construct the serrated recesses.
In yet other prior art devices employing projections or serrations within the recesses, the projections are spring loaded to bring more pressure against complementary grooves on a shaft for receiving the projections or the serrations on the internal surfaces of the holes, or, the serrations are helically shaped and adapted to be received in complementary helical grooves on the shaft. But both these devices are obviously complex in nature, require additional tooling and machining, and are costly to manufacture.
The improved cylindrical interference fit of the present invention, however, has several advantages over earlier fits for synthetic polymeric materials, in that a satisfactory three-point cylindrical force fit interlocking joint is surprisingly and unexpectedly effected with a smooth surface shaft and a recess having therein only two spaced, longitudinal ridges positioned within a 180.degree. angular segment of the recess, by which: a semi-permanent assembly free from accidental or undesirable separation is made possible; material costs and tooling are reduced; undesirable and harmful pressing forces which cause failure are substantially reduced; a need for additional hardware is altogether eliminated; and misalignment resulting from improper manufacturing tolerances of cylindrical interconnecting parts is substantially reduced.
Against the foregoing background, it is an object of this invention to provide an improved cylindrical interference fit for members molded from synthetic polymeric materials.
It is another object of this invention to provide three point cylindrical force fit interlocking joint which joint is of sufficient strength to hold synthetic polymeric constructed connecting members in self-sustaining assembly without allowing accidential or undesirable separation thereof and without a need for additional fastener components.
It is still an object of this invention to provide an improved three-point cylindrical force fit for synthetic polymeric material application while permitting optimum manufacturing economy.
It is a further object of this invention to provide an improved three-point cylindrical force fit for synthetic polymeric materials that substantially eliminates a need for refitting or remanufacturing of the intermating shaft, recess, or members themselves due to misalignments and improper tolerances.
It is yet another object of this invention to provide an improved three-point cylindrical force fit that freely permits simplified snap-on and snap-apart assembly and disassembly of interconnecting electrical terminal sectional blocks.
It is still a further object of this invention to provide a smooth surfaced shaft and ridged recesses for establishing a three-point cylindrical force fit interlocking joint for fastening a plurality of interconnecting synthetic polymeric constructed separate electrical terminal sectional blocks in a more or less permanent assembly of multiple electrical circuits without causing failure of the blocks during their assembly or disassembly.